Positron emission tomography (PET) with oxygen-15 water is widely used to determine regional cerebral blood flow (rCBF) changes during functional brain mapping experiments while subjects perform specific neurobehavioral tasks. We have performed simulation studies which showed that when rCBF changes during PET scan data collection, the measured flow is very close to the flow value present at the start of the scan, rather than being an average of the flows during the scan. This has implications with regard to the design and interpretation of brain mapping studies which involve two components, a learning or practice phase, and a recall or execution phase. If the two phases are performed in an alternating fashion during the scan without attention to when the scan starts, the measured rCBF will reflect a random combination of the rCBF profiles during the two phases. Instead, if task performance is coordinated with the time the scan starts, measured rCBF may reflect one or the other of the two phases, permitting one to determine which brain regions are involved in the task components. This hypothesis is being tested with a visual memory activation task which involves an encoding phase when subjects view slides of unfamiliar faces, and a recall phase when they indicate whether they have previously seen the faces presented to them. The injection time of 0-15 water and the onset of the task are selected so that subjects are performing one or the other of these two task phases during the first 20 sec of the scan. The scans should therefore reflect either encoding or recall. Scan data are currently being collected. Data will be analyzed to determine if different areas of activation are detected depending upon which phase of the task was being performed at the start of the scan. Results will also be compared to earlier experiments when this activation task was used but without regard to when the scan began.